Fused salt electrolyzer for magnesium production

ABSTRACT

An electrolyzer for the manufacture of magnesium, which has at least one electrolysis compartment with a cathode in the form of a frame whose members make up a closed loop around an anode in the electrolysis compartment. 
     The present invention relates to electrolyzers for the manufacture of magnesium. 
     Known in the art are electrolyzers for the manufacture of magnesium, comprising two compartments for the electrolysis of magnesium chloride, between which is placed a compartment to collect metallic magnesium. Between the electrolysis compartments and the collecting compartment there are partitions fabricated from refractory materials. The electrolysis compartments hold electrodes. Cathodes are provided in the form of plates with rods passing through a lining to a busbar. 
     However the above-mentioned electrolyzer has a low production rate. Attempts to increase the production rate of the electrolyzer by enlarging the electrolysis compartments reduce the current efficiency and entail increased power consumption per unit of the quantity of metal product. Attempts to improve the production rate of the electrolyzer by increasing its length results in requirement of extra shop floor area and inconveniences in servicing. The multitude of cathodes, with each cathode having a current-conducting connection of its own, passing through the lining, impair the strength of the lining at the fixation points. Another weak point about this type of electrolyzer lies in that the partitions made wholly of a refractory material. These partitions fail at a high rate, resulting in loss of chlorine. 
     The main object of the present invention is to eliminate the above-mentioned disadvantages. Other aims and advantages of the invention will become more fully apparent from the following description. 
     The object of the present invention is to provide an electrolyzer which offers a greater production rate, a high output of metal per unit area, improved utilization of the working volume, and simplified servicing, and has a reduced power consumption, fewer current-conducting connections built into the lining and an extended service life. 
     This objective is accomplished by providing an electrolyzer for the manufacture of magnesium, which, according to the invention, has at least one electrolysis compartment with a cathode in the form of a frame whose members form a closed loop around an anode in that compartment. 
     An electrolyzer of this type has a greater production rate. 
     It is expedient to make the members of the cathode (frame) in the form of walls and place the longitudinal walls between the electrolysis compartment and the collecting compartment in such a way that they constitute the lower portion of a partition. With this construction, it is seen that the output of metal per unit area is increased. 
     It is further preferable that the thickness of the longitudinal members of the cathode be made to increase towards the current-carrying connection. This improves the operation of the electrolyzer and provides a more uniform distribution of current in the cross members of the cathode frame. 
     A greater production rate is obtained if the frame-shaped cathodes are installed in at least three electrolysis compartments with at least two collecting compartments provided in between. 
     In a cell with three electrolysis .Iadd.compartments .Iaddend.the electrode in the middle compartment may be about twice as wide as the electrode in the outer electrolysis compartments. Thus better use is made of the operational area of an electrolyzer.

For a better understanding of the invention reference will now be madeto a preferred embodiment according to the invention and theaccompanying drawings, wherein:

FIG. 1 shows a cut-away view of an electrolyzer in plan, according tothe invention;

FIG. 2 shows vertical cross section II--II through the electrolyzer ofFIG. 1;

FIG. 3 shows vertical longitudinal section III--III through theelectrolyzer of FIG. 1.

Referring to FIG. 1, the electrolyzer has two outer electrolysiscompartments 1 and one middle electrolysis compartment 1', between whichare placed compartments 2 (FIG. 2) for collection of metallic magnesiumand slime. The electrolysis compartments 1 and 1' hold graphite anodes 3and cathodes 4, said cathodes being in the form of a frame or a closedloop, each loop consisting of two longitudinal members 5 and severalcross members 6. The width of the longitudinal members 5 decreases inthe direction away from the current-conducting connection, which factprovides for a more uniform distribution of current in the cross members6 of the cathode frame.

For directed circulation of the electrolyte and better transfer of themetal (magnesium) from the electrolysis compartments into the collectingcompartments, the cross members 6 of the cathode frames (which are themain working surfaces of the cathodes) have bevelled portions 7 whichincrease the interelectrode distance in the direction of the collectingcompartments 2. The electrolysis compartments 1 and 1' are placed incommunication with the collecting compartments 2 by top and bottom slots8 (FIG. 3) in the partitions whose upper parts 9 (FIG. 2) are made of arefractory material. The lower portion of each partition is also one ofthe longitudinal members 5 of the cathode frame. The middle electrolysiscompartment 1' and the electrodes located in the compartment arerespectively twice as wide as the outer compartments 1 and theirelectrodes. The anodes are embedded in the brickwork of the electrolyzerbottom 10. Current to the anodes is conducted from beneath, through acast-iron contact slab 11 and an aluminum shank 12. The longitudinalmembers of the cathode frames extend through the electrolyzer lining 13to a d.c. source.

The electrolyzer operates as follows. The bath is filled with anelectrolyte consisting of molten chlorides of alkaline and alkalineearth metals to a level covering the slots 8 in the partitions. Thendirect current is applied to the electrodes, and the electrolysis of themagnesium chloride begins. The result is that magnesium and chloride areliberated at the respective electrodes. The operation is carried on atan electrolyte temperature of 680°-730° C. The electrolyte rises to thesurface and carries the magnesium and chlorine along; the chlorineleaves the electrolyte and collects at the top of the electrolysiscompartment. Chlorine removal from the electrolyzer is effected througha port in the top cover of the electrolyzer. The metal-electrode mixturefreed from chlorine flows from the electrolysis compartments through theslots 8 in the lower portion of the partition into the collectingcompartment. The magnesium metal collects at the top of the collectingcompartment, while the electrolyte flows through the lower portion ofthe partition back into the electrolysis compartment. In this way, thereis a closed flow of electrolyte providing for continuous transfer of themetal from the electrolysis compartments into the collectingcompartment. In the outer electrolysis compartments 1 the closed flow ofelectrolyte carries the metal in one direction, towards the collectingcompartment. In the middle electrolysis compartment, located between twocollecting compartments, a closed flow of electrolyte carries the metalin opposite directions, into the two collecting compartments. Therefore,the conditions for the transfer of the metal from the middleelectrolysis compartment which is nearly twice as long as the outer onesare the same as for the transfer from the outer compartments.

In the electrolysis compartment the spacing between the electrodesgradually increases in the direction of the collecting compartments.Therefore, the cell area farthest from the collecting compartments havean increased current density and a maximum gas content in theelectrolyte, which factors provide for directed circulation of theelectrolyte essential to the transfer of the metal to the collectingcompartments across the entire width of the electrolysis compartments.

The metal is removed from the collecting compartments at regularintervals, as it collects there. The slime from the electrolyte settlingat the bottom is also removed periodically.

The electrolyzer described above has been tested. The energy consumptionhas been 12.5 kWh per kilogram of magnesium, which is 20 to 25 percentless than in the case of other existing electrolyzers.

In other embodiments of the proposed electrolyzer, the longitudinalmembers of cathode frames may be of uniform cross-section along theirentire length. Variations in the electrode spacing in the electrolysiscompartment can be obtained by using anodes with bevelled faces in thedirection of the collecting compartments. Also, current to the anodesmay be supplied from above.

What is claimed is:
 1. An electrolyzer for the manufacture of magnesium,comprising in combination: at least three electrolysis compartmentshousing electrodes; and at least one magnesium collecting compartmentlocated between said electrolysis compartments each of the electrolysiscompartments having a plurality of anode members and a cathode in theform of a frame whose members form a closed loop around each anodemember in said electrolysis compartment.
 2. An electrolyzer, as claimedin claim 1, which includes partition walls between electrolysiscompartments and collecting compartments and in which said cathodemembers comprise longitudinal members which are made in the form ofwalls which form a lower portion of the partition walls.
 3. Anelectrolyzer, as claimed in claim 2, in which the thickness of saidlongitudinal cathode members increases in the direction of thecurrent-conducting connection thereof.
 4. An electrolyzer, as claimed inclaim 1, having three said electrolysis compartments, in which anelectrode located in the middle electrolysis compartment issubstantially twice as wide as the corresponding electrodes located inthe outer electrolysis compartments. .Iadd.
 5. In an electrolytic cellfor melt-electrolytic production of magnesium, comprising electrolysisand collecting compartments, said electrolysis compartment containingcathodes and anodes, the improvement comprising each of said cathodesconsisting of two longitudinal members and a plurality of transversemembers, said members forming a closed frame around at least one anodein said electrolysis compartment. .Iaddend.